TellApart’s customers include many of the Web’s biggest retailers, including CSN Stores, which operates the largest online-only home goods store through a network of more than 200 sites including Luggage.com and Cookware.com. The company had $380 million in sales last year and is expanding internationally. Jeff Steeves, director of customer acquisition at the company, says it has worked with TellApart since last May. “We see a significant lift to our conversion rate,” he says. “They do the heavy lifting for us, operating models to measure a user’s engagement and determine … which are most likely to make a purchase if shown a targeted ad.” The company doesn’t want to irritate unlikely buyers or spend money on ads that are unlikely to lead to a sale. “Our breed of remarketing is subtle,” says Ayzenshtat.

All this information gives TellApart a very detailed picture of a site visitor. The retailer does not, however, disclose to TellApart a person’s name or username. “We’re not interested in that,” says Ayzenshtat. The CEO of another retargeting company, who did not want to be named for this story, noted that user privacy is mostly self-regulated by the big players in the online ad industry, especially Google—and retargeters would like to keep it that way rather than have the U.S. Federal Trade Commission, for example, get involved. One of the principles of this self-regulation is that users have to be able to opt out of an advertising technology. TellApart satisfies that requirement by putting a little “x” in the corner of ads it shows, though it says only a tiny fraction of the people it targets have opted out.

Retargeting is likely to get even more precise. Ayzenshtat says TellApart has determined that it can drive a 30 percent increase in sales by picking out which consumers are more likely to buy something when presented with a coupon, and which ones don’t need coupons as an inducement. Similarly, some sites offer an opportunity to put in a coupon code when checking out; some people who don’t have one will simply ignore it and follow through on a purchase, but others will go out and search the Web for mentions of that code, find it, and then return to complete their transaction with a discount of, say, 20 percent. Now TellApart is developing a way for retailers to know when to display a place to enter a coupon code, and when they don’t need to bother.